P1.8P1. Motion, Forces and Energy
Pressure — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 14 of 15 1 quiz
Lesson summary
P1.8 Pressure — COMBINED SCIENCE notes: Define pressure, use p = F/A, and explain how pressure in a liquid depends on depth and density.Pressure = the force acting perpendicular to a surface per unit area.p = F ÷ A, with p in pascals (Pa), F in N and A in m².What the symbols mean…
- Rearranged forms
- Area conversion
- Several contact points
- Same force, smaller area
- Same force, larger area
- All directions
Section 1 of 8Pressure
Pressure
Pressure the force acting perpendicular to a surface per unit area.
- p = F ÷ A, with p in pascals (Pa), F in N and A in m².What the symbols mean: p = pressure (Pa = N m−2) · F = force acting at right angles to the surface (N) · A = area (m2)
Unit
- 1 Pa = 1 N/m², and 1 kPa = 1000 Pa.
Rearranged forms
- F = p × A and A = F ÷ p.
Area conversion
- 1 cm² = 1 × 10−4 m², so divide cm² by 10 000 to get m².
Several contact points
- Several contact points add their areas to find the total area.
Section 2 of 8Effect of area on pressure
Same force, smaller area
- Same force, smaller area the pressure is larger.
Same force, larger area
- Same force, larger area the pressure is smaller.
| Object | Area | Effect |
|---|---|---|
| Tractor with wide tyres | Large | Low pressure, so it does not sink into mud |
| Drawing pin point | Very small | High pressure, so it pushes into a wall |
| High-heeled shoe | Small | Higher pressure than a flat shoe, so it sinks into soft ground |
| Knife edge or needle | Very small | High pressure, so it cuts easily |
Section 3 of 8Pressure in a liquid
Cause
- Cause the pressure in a liquid is caused by the weight of the liquid above the point.
Depth
- Depth pressure increases with depth, so the highest pressure is at the bottom.
- Water spurts out harder and further from a hole near the bottom of a container.
Density
- Density a denser liquid exerts more pressure at the same depth.
- Sea water is denser than fresh water, so it gives a greater pressure at the same depth.
All directions
- All directions at a given depth the pressure acts equally in all directions and at right angles to every surface.
Dams
- Dams they are thicker at the bottom because the water pressure is greatest at the greatest depth.
Section 4 of 8Calculating liquid pressure
Equation
- Δp = ρ × g × Δh.What the symbols mean: p = pressure due to the liquid (Pa) · ρ = density of the liquid (kg m−3) · g = 9.81 N kg−1 · h = depth (m)
- Δp is the change in pressure in Pa and ρ is the density in kg/m³.
- g is the gravitational field strength in N/kg and Δh is the change in depth in m.
Rate of increase
- Rate of increase in water the pressure rises by about 10 kPa for every metre of depth.
Total pressure
- Total pressure the pressure at a depth in a liquid is the atmospheric pressure plus ρgh.
Section 5 of 8Atmospheric pressure
Cause
- Cause the weight of the air above a surface.
Size
- Size about 100 kPa at sea level.
Height
- Height it decreases with height because there is less air above.
Section 6 of 8More from the specification
More from the specification
Normal perpendicular contact forces
- Normal perpendicular contact forces fluid pressure exerts uniform perpendicular compressive forces acting evenly across every square metre of submerged surfaces.
Pressure
- Pressure force per unit area, p = F/A, measured in pascals (N per m2)
Effect of area
- Effect of area the same force on a smaller area gives greater pressure, which is why high heels sink into soft ground and knives cut well
Spreading force
- Spreading force wide tyres or snowshoes spread the weight over a larger area, reducing pressure
Liquid pressure
- Liquid pressure p = h x density x g, so pressure increases with depth and density
Measuring pressure
- Measuring pressure a column of mercury or a manometer measures pressure differences, with relevant quantities being height, density and gravitational field strength
Pressure
- Pressure force per unit area, p = F/A, measured in pascals (N per m2)
Effect of area
- Effect of area the same force on a smaller area gives greater pressure, which is why high heels sink into soft ground and knives cut well
Spreading force
- Spreading force wide tyres or snowshoes spread the weight over a larger area, reducing pressure
Liquid pressure
- Liquid pressure p = h x density x g, so pressure increases with depth and density
Measuring pressure
- Measuring pressure a column of mercury or a manometer measures pressure differences, with relevant quantities being height, density and gravitational field strength
Newton
- The unit of force.
Spread
- The difference between the smallest and largest repeated measurements.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Pressure | The force acting perpendicular to a surface per unit area. |
| Pascal | The unit of pressure, equal to 1 N/m². |
| Liquid pressure | Pressure caused by the weight of liquid above a point. |
| Density | Mass per unit volume of a material. |
| Atmospheric pressure | The pressure caused by the weight of the air above. |
| Depth | The vertical distance below the surface of a liquid. |
Section 8 of 8Exam technique
1
Using cm² directly in p = F/A — convert to m² first.
2
Saying liquid pressure depends on the shape of the container — it depends on depth and density.
3
Saying pressure acts only downwards in a liquid — it acts equally in all directions at a given depth.
4
Confusing force with pressure — pressure is force per unit area.
5
Using total depth of the container instead of the depth below the surface.
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